1. Field of the Invention
This invention relates to an apparatus and a method and process for forming and pouring manhole bases. The process of the invention allows manhole bases to be made watertight by allowing for the vibration of the setting concrete to eliminate entrapped air. In this manner, water porous honey-combed concrete is thereby avoided. Using the process of the invention, pouring, in accordance with this invention, can be done in one application, instead of the usual mutiple cycle of pouring, setting and re-pouring. This one-step pouring results in a wall of uniform colour, and the resulting smooth side walls facilitate water or sewage flow. Wavy-shaped walls that bulge into the water channel and cause flow restrictions are avoided. Cleaning equipment functions better in the smooth sided manhole assemblies made according to the process of the invention. The form materials and apparatus of the invention can be re-used and are thus not wasted. Pipe stub channels need no longer be crudely sculpted by hand, in the setting concrete.
2. Description of the Prior Art
The standard method for erecting a manhole is to first dig the required excavation. A precast manhole barrel is delivered to the work site and workmen notch the base of the barrel to accommodate pipe stubs. The pipe stubs connect with pipes leading into and away from the manhole assembly. In order to make the notch, the manhole barrel wall is notched with a sledge hammer and the reinforcing mesh wire is cut away. The resulting notches fit over the pipe stubs which are pieces of pipe that protrude into the manhole or through its,--if the water course is to be a straight path. The pipe stub portion that protrudes into the manhole has its top half removed. The water passes through the manhole by entering a pipe stub then passing through a connecting channel in the manhole's concrete floor and leaving through a connecting pipe stub.
The pipe stub is set on brick skims that are carefully placed to allow for proper grade and alignment.
The earth is filled or excavated to allow for about a 6 inch clearance under the stubs. Bricks are also piled up in order to support the precast manhole barrel. This barrel is lowered by a backhoe or other means onto the bricks, with the barrel's notches fitting over the pipes or pipe stubs. Concrete is then poured into the centre of the manhole and it is worked out to the periphery of the excavation by workmen. Vibration, to remove entrapped air, is used sparingly or not at all because lack of forms enables the concrete to escape down the pipe which becomes difficult to retrieve. The channel between pipe stubs is made by hand-scooping, and rough-shaping of the benching. It is almost impossible to achieve required uniform shape and size in manholes that require compound curves. Hand-shaping is just too crude. Workmen must usually rough bench the initial pour, allow it to set and then build up the required benching with successive pours of concrete. This method results in unsightly, multicoloured layers of concrete, and the stratified vertical walls are usually wavy and tend to bulge. The bulges can impede water flow, cause restrictions and may impede cleaning equipment. Patching that is less than 2 inches thick can crack in warm weather and chip off due to subsequent, normal water flow.
Concrete that is not properly vibrated, tends to be honeycombed and may not be water tight, necessitating subsequent patching with mortar.
The bricks supporting manhole barrels in excavations often sag when the bricks sink into the soil causing the manhole assembly to tilt. In assemblies over 10 feet, the tilt is noticeable and cause the access ladders to become tilted from vertical. Subsequent rectification efforts are expensive, and time consuming. The pool of workmen skilled in pouring and sculpting concrete manhole bases are rapidly diminishing.
Some contractors use plywood (usually 3/4 inch) to line the excavation's outer limits and to shape the channels. These forms, though an improvement, require considerable time to cut, fit and brace into place. The form is usually discarded, after the job, because manholes are rarely uniform and are usually unique to each excavation. The plywood form is often damaged during its removal and is rendered unusable for future jobs. Plywood forms generally cannot withstand the pressures attending concrete vibration, thus, concrete vibration is deleted or used sparingly. Many work sites have very poor soil that cannot support wheeled concrete delivery trucks making in situ manhole construction impossible.
Some manhole assembles are prefabricated, and shipped to the work site. These assembles are made with a one foot barrel and a poured concrete base. Fibre glass forms are used for the purpose of concrete pouring. The size of the manholes are thus limited in variety and there are problems fitting the standard connections in situ.